Ethernet devices include pulse transformers for the purpose of achieving impedance match and electrical insulation in input/output terminals. Such transformers include magnetic cores generally composed of soft magnetic materials. Such pulse transformers are required to have a high incremental permeability μΔ under the application of a direct-current magnetic field in a temperature range of −40 to 85° C., for example, as defined in American standards ANSI X3.263-1995[R2000]. The incremental permeability μΔ is a value that indicates the degree of magnetization of a magnetic core under the application of a magnetic field.
A soft magnetic material used in the application is generally Mn—Zn ferrite. Mn—Zn ferrite is advantageous, for example, in that a high permeability and a high inductance can be easily achieved for a soft magnetic material and Mn—Zn ferrite is less expensive than amorphous metals and the like. Developments of Mn—Zn ferrites suitable for the application are performed and described in, for example, Patent Literatures 1 and 2.
However, Mn—Zn ferrite is an oxide magnetic material and hence has drawbacks in that, compared with metal magnetic materials, magnetic characteristics considerably vary with change in temperature and a material having a high permeability has a low saturation flux density.
Accordingly, stable magnetic characteristics are less likely to be achieved in a wide temperature range, in particular, in a high temperature range, which is problematic.
To overcome the temperature dependency of characteristics of Mn—Zn ferrite, it is known that addition of CoO having positive magnetic anisotropy is effective. For example, Patent Literature 1 states that a Mn—Zn—Co ferrite for the application allows a high permeability under the application of a direct-current magnetic field of about 33 A/m.
The inventors previously developed “A ferrite core comprising a basic component, additional components, and impurities, wherein the basic component consists of Fe2O3: 51.0 to 53.0 mol %, ZnO: 13.0 to 18.0 mol %, CoO: 0.04 to 0.60 mol %, and the balance being MnO; the ferrite contains, as the additional components, SiO2: 0.005 to 0.040 mass % and CaO: 0.020 to 0.400 mass % relative to an entirety of the ferrite; the ferrite contains, as the impurities, P: less than 3 mass ppm and B: less than 3 mass ppm relative to the entirety of the ferrite; and an average pulverized particle size is 1.00 to 1.30 μm”, which is suitable as a magnetic core for a pulse transformer in an Ethernet device, and disclosed the ferrite core in Patent Literature 3.
The development of such a ferrite core allows a high incremental permeability of 2300 or more under the application of a direct-current magnetic field of 33 A/m in a wide temperature range of −40° C. to 85° C.
However, a Mn—Zn—Co ferrite core in the application mainly has the form of a closed magnetic circuit with a small size represented by a toroidal core having an outer diameter of about 2 to 6 mm. In the case of such a small size, since there is a high probability of mold breakage in compaction, it is impossible to apply a high compaction pressure. Accordingly, when a surface of a core that is a fired compact as illustrated in FIG. 1 is observed with a scanning electron microscope (SEM), there are cases where cavities due to insufficient disintegration of granulated powder remain as illustrated in FIG. 2(b).
When a core includes such cavities, the volume occupied by the magnetic material is small and hence magnetic fluxes concentrate in the magnetic material region and a magnetic flux density locally increases. Accordingly, the same phenomenon as an increase in a superposed magnetic field seemingly occurs in the magnetic material region and, as a result, the incremental permeability decreases. Thus, it is difficult to continuously maintain an incremental permeability of 2000 or more in a wide temperature range of −40° C. to 85° C.